Sap temperature impact on RO

[paulslund]

I built an RO with 2 x 400gpd membranes and a proper 400gpd RO pump from the Coronwater store (although it's not a coronwater pump, it still flows 42gph @80psi, so flow is no issue). When I flushed with cold water and did a secondary test with cold tap water (long story on why I did this second test) I was getting about 8gph of permeate @ 110psi which is what I was expecting, and combined flow of around 24gph of permeate/concentrate at this pressure.

Fast forward to last weekend when actually processing sap, and the performance decreased significantly... down to ~ 2gph, and that was cranked up to 130psi just to get that flow.

The SAP was cold.. had tons of ice in the barrel and likely stayed cold for most of the RO process as it was cold/snowing outside.. so likely not much above freezing. I did have a heater around the RO itself to keep the system warm while processing.. I don't think it helped performance but I kept it from freezing as I ended up running it all night to bring down as much as possible for the following day's boil...

Post RO flushing with cold permeate provided combined flow of ~42GPH (the "natural" pressure of the tubing at 30psi provided about 4.8gph of new permeate).

I'm going to test each membrane separately to ensure no blockage, but was my issue the temperature of the sap that cause me to only have about 25% of the performance I expected? Does going from the recommended 4-5C down to almost 0-1C make that much of a difference?

The irony is that I obtained a large fridge to keep sap cool during those warm spells, but this seems to go against what I need to do to get the right performance out of the RO.

Has anyone done an actual comparison with sap temps?

Any insight would be appreciated!

Thanks!
Paul!

[billyinvt]

Sap processes at about half the rate of water. So a 400gpd membrane, will do 200gpd. About 8 gph, 4 concentrate/4 permeate. Higher psi actually decreases flow rate because to get that pressure you are restricting flow back to the membranes. You end up with a higher ratio of concentrate to water, but it takes longer. And yes, colder sap processes slower. Processing rates on household membranes are typically based on 40 degree liquid, which is the approximate temperature of municipal and well water.

[paulslund]

When I was researching what size to build I found a general guideline that one can expect to take the rated GPD and get about 25% of it, accounting for increased PSI vs decrease temp and increase TDS of sap compared to water, so I expected approx 4gph per membrane (although I know realistically was more likely to be around 7ish to account for the increased TDS from concentrate running into membrane # 2)

I'm measuring performance strictly on the permeate flow, not the overall flow. I was expecting approx 8gph (or 7) of permeate creation @ 100PSI, and ended up with only approx 2gph @130PSI with the really cold sap. Higher PSI should create faster permeate, but I agree that the overall rate of flow will decrease as a result.. which is fine.

So I'm just curious if anyone has tested the difference that very cold sap makes and if this is a normal/expected reduction due to temperatures...

[wnybassman]

With my RO I know 33 degree sap feels painfully slow compared to 50+ degree sap. Huge difference for me.

[DrTimPerkins]

RO machines/membranes are typically rated at 77 deg F for water. Temperature has a huge effect on membrane efficiency and sap processing rate. Not unusual for actual (good quality) SAP flow rates at 35-40 deg F to be only about 1/3 (or less) that of the rated value. If your membranes are somewhat fouled or sap quality poor it can be even less.

[paulslund]

RO machines/membranes are typically rated at 77 deg F for water. Temperature has a huge effect on membrane efficiency and sap processing rate. Not unusual for actual (good quality) SAP flow rates at 35-40 deg F to be only about 1/3 (or less) that of the rated value. If your membranes are somewhat fouled or sap quality poor it can be even less.

So that's inline with my estimated 25% per membrane and 8pgh of permeate combined.. but I ended up with 2gph, with sap that was likely only around freezing temps I suppose.. I wish I could have taken the sap temperature.. These were new membranes that had only been flushed... I flushed them with the permeate after processing and at 30psi (needle valve open, but natural restrictions from tubing) I was getting around 4-5pgh of permeate.. obviously the input is permeate so this is to be expected..

I suppose it could be a combination of sap temperature and high TDS sap perhaps? Oh well.. I will test it again in a few days.. unfortunately we have a cold spell coming in just when I need to run it again and boil.. :mad:

[MajorWoodchuck]

Make sure you have enough flow going through the membranes. I try to just run 100 psi and keep the concentrate flow 5x that of permeate peon my similar setup (2x 400) and get at least 5 GPH water out of them. I try to get my sap closer to 35-40 degrees when it goes through the RO. I RO overnight taking 50% water out of he barrels i am recirculating it through. Then run it again to feed my evaporator. i have a copper line inside my stack that preheats the sap to almost boiling. i can adjust the recirculation valve to keep the flow not to fast that it stops the boil in my hottest pan but not too slow that it starts to spit out sap vapor. I want to try experimenting running a copper line against my uninsulated boiler to preheat the sap a little before it hits the RO. i am just nervous about getting it too hot and ruining my membranes.

[paulslund]

I want to try experimenting running a copper line against my uninsulated boiler to preheat the sap a little before it hits the RO. i am just nervous about getting it too hot and ruining my membranes.

I saw a youtube on exactly that.... they made a copper coil that was attached on the back of the stack, and just hung there so that it could be removable.. I'll try to find it.

So at 100PSI you are getting around 5GPH... hmm.. I thought my research showed 2x400 should be getting around 8, but maybe that's if all the stars are properly aligned.. :D and 5 is more reasonable..THanks!

[paulslund]

So I've kept my sap from freezing in this cold spell and even kept the sap area around 7degrees (give or take) to make sure the sap isn't too cold.. unfortunately I'm still only getting around 2 GPH of permeate, although at a slightly less 120PSI.

Perhaps it is the membranes themselves aren't suitable to handle sap? I bought them on AliExpress.. but I can't seem to find a rating on them in terms of TDS they can effectively handle..(not sure if I've ever noticed that on any membrane stats, to be honest..)

[DRoseum]

Possible it's the membrane brand. I've had really good success with Membrane Solutions brand.

[paulslund]

Possible it's the membrane brand. I've had really good success with Membrane Solutions brand.

I just checked the specifications and they seem to be the same (i.e micron size, lower psi though, which I thought would be beneficial), but things aren't always created equal I suppose. I'll try different ones next season.. I think we are almost done at my end..

[paulslund]

So I seem to remember a post (that I can't find anymore) that mentioned the water filter itself could pose an issue with flow.. The ones I bought are described as being made of "fine melted polypropylene that is blown and molded into a cylinder like form".. which I thought was fine, but as I think back to my test with the carbon filter, flow was much greater than even my water test with this 5 micron filter. Flushing with permeate flowed quite high (although not as high as the pump specs but stilla bout 42gph, but permeate would have no resistance I imagine..?)

Could this be the cause of my flow issues?

https://www.amazon.ca/gp/product/B01M01EDHK/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&th=1

[paulslund]

So I'm ruling out the water filter as I've seen other posts using the same filter. Permeate production was even worse today than last night down to about 1.5GPH, and when flushing with permeate the PSI stayed at 60, and went down to about 50 while flushing.. instead of the normal 30.. I'm guessing the membranes either can't handle the TDS of sap, or they were somehow badly fouled when I got them (they only have about 200 gallons through them).

At least it was a beautiful sunny day for boiling today! :-)

[Mean_Oscar]

At 33 F, you may have frozen and thawed sap resulting in an up to 12% sugar concentration at the bottom of the vessel. Put the intake in that and it will stop a 6% capable system in its tracks.